1. Field of the Invention
This invention relates to an apparatus for the suspension smelting of finely-grained oxide and/or sulfide ores and concentrates.
2. Prior Art
In this connection it must first be noted that literature usually gives very little information concerning the solid and smelted materials carried along in a suspension by gas flows in smelting furnaces and other corresponding pyrometallurgical processing units. Furthermore, the available information usually relates to the amount and separation of dusts emerging from the apparatus but very seldom to the behaviour of the sediments of material inside the apparatus and their effect on the process.
In the conventional reverberatory furnaces which are used in mass production of sulfide processing and which can be considered a distant origin of the actual suspension smelting systems, a considerable part is played in dust formation. In order to eliminate dust formation in these appaaratuses, hot feeding is replaced by wet feeding which prevents the entrance into the apparatus of very fine, dust-producing material formed particularly in the roasting process. The latest development in reverberatory furnace systems is the so-called Worcra smelting system (West-German Pat. No. 1,533,061 or Canadian Pat. No. 814,926). In order to reduce the amount of dust, this process uses pellets or agglomerates of a concentrate mixture instead of loose concentrates. In regard to the conventional reverberatory furnace smelting, the feeding method has also been changed for the same reason so that the previous feeding along the walls over almost the entire length (60-80%) of the furnace has been replaced by feeding within a zone in the middle part of the furnace usually consisting of less than 30% in the furnace length. In spite of these changes, the amount of dust (flying dust) is still about 3% of the weight of the feed. It can be supposed that, especially in a linear reverberatory furnace type, even a great amount of dust suspended in the gas phase will not, in the form of a furnace sediment, cause considerable increase in the valuable metal content of the slag because the gas phase will carry the dust along when it moves over the feeding bed. The valuable metal content and its increase in reverberatory furnace slag is thus mainly due to the flow phenomena caused by the system itself or by the feeding of additional materials -- for example, converter-slag feed. The actual difficulties due to the dissolution of the suspension, dust sedimentation inside the furnace systems, and high amounts of flying dust are created when using actual suspension smelting systems.
In the suspension smelting process developed by the International Nickol Company (U.S. Pat. No. 2,668,107), in which horizontal spraying is used, great amounts of dust are eliminated by adjusting the concentrate granule size suitable for the process so that the granules are considerably large (U.S. Pat. No. 2,668,107, granule size between 95%/-65 mesh and 50%/-200 mesh, and B.D.P. 840 441, granule size between 95%/-65 mesh and 5%/-200 mesh). Owing to horizontal burning, the amount of dust which sediments inside the system of the said process is so great that even when rough concentrate is used the slag obtained from the furnace is so rich in valuable metals that its after-treatment has been combined with the main process. Concentrate is produced in the process by suspension smelting matte (a/57.65% Cu and 3.03% Ni, and b/6.95% Cu and 33.77% Ni) during the first operation period -- for example, 6 hours. After the removal of the matte, the slag phase (a/0.90% Cu and 0.15% Ni, and b/0.15% Cu and 0.60% Ni) remaining in the furnace is "washed" and the valuable metals by suspension smelting pyrrolite (a/1% Ni and b/1.25% Ni) during the second operation period -- for example, 1-2 hours -- and not until then is disposable slag (a/0.35% Cu and 0.16% Ni, and b/0.12% Cu and 0.30% Ni) obtained and, in addition, poor matte is a side product.
It must particularly be noted that, in spite of the use of rough concentrate, the amount of flying dust in the exhaust gases is very great, about 5% of the amount of the feed, even though the gas volumes and obviously also their flowing velocities are very low owing to the pure oxygen or oxygen-rich air used in the burning of concentrate -- the SO.sub.2 content of the exhaust gases is 85%.
The flash smelting system developed at Outokumpu Oy and its modifications (U.S. Pat. Nos. 2,506,577 and 3,306,708) include vertical burning of suspended concentrate. In this case, when rough concentrate is used, the solid and smelted materials will not cause furnace sediment problems corresponding to those created in the horizontal process.
The valuable metal contents of the slags of the vertical process are also respectively quite low, and no special aftertreatment is usually needed unless unsuitable additional materials are either fed together with the concentrate or enter the system by other routes. The basic process (Finnish Pat. No. 22,694, U.S. Pat. No. 2,506,557) contains no mention of dusts. In the pyrite treatment process (Finnish Pat. No. 32,465, U.S. Pat. No. 3,306,708), in which sediment dust has no practical importance, the effect of the flying dust amount, about 2-3%, is also technically insignificant. In the known processes for the oxidation and reduction of suspension, the amount of dust in the indutrial-scale process is very small in comparison with the used gas amount, or 4-6% of the weight of the feed mixture. It can, however, be noted that the sediments inside the system corresponding to even these small dust amounts, obtained with relatively rough concentrates, have an effect on the valuable metal contents in the slag. However, actual dust problems appear with concentrates more finely-grained than usual; the problems being only when the vertical suspension can be made to dissolve only partially. Of new modifications of the suspension process, let us mention the Brittingham process (U.S. Pat. No, 3,460,817) in which an attempt has been made to combine the developed reverberatory furnace process producing raw copper as the final product according to the previously mentioned Worcra process, and the vertical-burning smelting process according to the Outokumpu process. Furthermore, to the system so obtained has been added (U.S. Pat. No. 3,668,107) a sulfide suspension slag wash analogous to the Inco process, but the periodical, shaftless horizontal suspension burning of the latter has been replaced by a continuous-working vertical-burning additional shaft. A smoke tower has been placed between the reaction shaft and the additonal shaft. Thus, the meeting of gas flows from opposite directions ought to make the solid or smelted materials contained in them to fall. There is no mention of the actual product sedimentation rates and flying dust amounts. Because the process has not been applied on an industrial scale, it is difficult to evaluate without any operational information the behavior of the system in regard to these factors.
According to one known process (Canadian Pat. No. 760,925), copper concentrate and additional fuel are injected with the help of compressed air (normal or rich in oxygen) inside smelted ore at a high temperature, at which time the partially oxidized concentrate is arrested in the smelt and smelts forming matte and slag. On the other hand, the additional fuel ought to burn in the smelted material, supplementing the amount of heat required for smelting the ore and, thus, the smelt should always remain at a constant temperature.
According to the process, the copper contents of the mattes are 40.5, 66.9, and 39.4%, and the copper and sulfur contents of the respective slags 0.33, 0.52, and 0.35% Cu and 0.20, 0.23, and 0.26% S. Theoretically, the known balances between the matte and the slag do not materialize with the contents given, especially in regard to sulfur. The obtained result deviates especially from results obtained from reverberatory and flash smelting furnaces. When comparing the copper contents of slags obtained by vertical suspension smelting to the values given above, it can be noted that when the burning of iron is only about 28%, the concentrate and matte contents being 27.8% and about 40% Cu, the values given are by no means rare. It does not give the ferric iron and magnetite contents of the slags so that a comparison with the flash smelting process is without foundation.
On the basis of the balances, no flying dusts are created in this known process, which is very rare in suspension processes. When drafting a heat balance on the basis of the material balances, it is noted that practically at the lowest operational temperature than can be considered, 1250.degree. C, the obtained difference between the incoming and outgoing temperatures of the system is zero, so that according to this the used apparatus would have no heat losses (balance references Cu.sub.2 O and FeO: into the system: feed mixture 960 kg/824 Mcal and kerosene 40 kg/404 Mcal, or a total of 1227 Mcal; out of the system: matte 635.4 kg/672 Mcal, slag 231.4 kg/80 Mcal, and gas phase (9.4% SO.sub.2) 1025 Nm.sup.3 /474 Mcal, or a total of 1226 Mcal. Difference: 1 Mcal). The heat load values cannot be estimated or compared with other processes because the dimensioning of the apparatus, the delay period, and the capacity values have not been given.
Finally we will review an older suspension roasting process (U.S. Pat. No. 2,209,331), in which dust problems have been discussed more than the newer processes.
According to the process, materials containing sulfides are roasted or roasted and smelted, and the sulfur content of these materials is recovered preferably in the elemental form or in the form of gases with high SO.sub.2 contents. In the process, the roasting is accomplished by dispersing a finely-grained sulfidic material into an oxidizing gas flow which consists of oxygen or oxygen-rich air. After the roasting the material is recovered either in a solid state or in the form of a molten bath. The amount of free oxygen in the oxidizing gas can be regulated so that the free oxygen is used up by oxidizing only part of the sulfur content of the sulfidic material into SO.sub.2. The temperature is raised so high tha the sulfide smelts in suspension. The iron sulfides have been meant to react to a considerable degree with SO.sub.2 to form iron oxide and free sulfur as soon as the oxygen has been used in the formation of SO.sub.2.
The processes take place in a vertical reaction tower, in which case the following processes, among others, can be used:
-- Cocurrent process: A sulfidic material and an oxidizing gas are fed downwards and the product is separated from the gases with the help of a small gas volume and low flowing velocities. The sulfidic material and the oxidizing gas are fed upwards and the velocity is adjusted so that as great a part as possible of the product is carried upward by the gas flow and separated after the tower by known methods. In this case, rough granules will fall countercurrently to the bottom of the tower and are reoxidized when needed.
-- Countercurrent process: An oxidizing gas flow is fed upwards and the material to be roasted, downwards. The process is particularly suitable for sulfidic material in which the amount of extremely fine particles is not too great.
-- In certain cases it is recommendable according to the process to divide the sulfidic material into different granule categories and treat each of them separately, and the most finely-grained particles preferably by a parallel flow.
The dissolution of the suspension in the process is thus mainly based on the high density of the suspension and very low gas velocities, in which case the settling circumstances mainly determined by the principle of Stokes are obviously achieved especially when using relatively rough granule distributions.
Even though the described process has mainly been meant for the roasting of iron sulfides, it can, according to the specification, also be used for the treatment of sulfides of valuable metals in the manner described above. Valuable metals Cu, Ni, and Pb are separated from the suspension either as a metal smelt (Cu, Pb), a sulfide smelt, or a powder; in the latter case other known separation processes (crushing, grinding, foaming, magnet separation, chlorination, etc.) are used for the refinement and metal separation.
The object of the present invention is to create an apparatus for the suspension smelting of very finely-grained oxide and/or sulfide concentrates by the flash smelting process into sulfide mattes with high or low valuable metal contents, the respective slags being very poor in valuable metals. The apparatus thus also allows for economical processing of finely-grained concentrates by the suspension process, and characterizing of the invention is that in the lower furnace, between the main smelt reaction zone and the rising-flow zone, there is also secondary smelt reaction zone in which the residual suspension may at least partially dissolve in smelt before essentially all undissolved residual suspension is fed into the rising-flow zone, and in the lower furnace there is also a separate smelt settling zone communicating at least through the smelt with the main and secondary smelt reaction zones for the separation of slag from matte and metal and provided with devices for removing slag, metal, and matte from the lower furnace.